Process for the production of terephalic acid

ABSTRACT

A terephthalic acid slurry in acetic acid is produced by oxidizing p-xylene in acetic acid, removing water by evaporation of a stream of water and acetic acid, and returning acetic acid to the oxidation step. The terephthalic acid is separated from the reaction medium in a first zone to leave a deposit on a band, the deposit is washed with a first aqueous medium in a second zone, removed from the band in a third zone, and admixed with a second aqueous medium. Reaction medium is passed from the first zone to the oxidation step and terephthalic acid is recovered, preferably after further purification.

This is a continuation of application Ser. No. 08/470,955, filed Jun. 6,1995 now U.S. Pat. No. 5,563,293 which is a continuation of Ser. No.07/847,735, filed Mar. 9, 1992.

This invention relates to a process for the production of terephthalicacid.

Terephthalic acid is produced commercially by oxidising para xylene withoxygen in a liquid phase which comprises acetic acid and a dissolvedcatalyst. The temperature of the reaction and the concentration ofwater, which is produced as a by-product, in the liquid phase iscontrolled by withdrawing from the reaction as vapour a streamcomprising water and acetic acid. Water may be removed from at leastpart of the stream leaving a stream enriched in acetic acid which may bereturned to the reaction.

The terephthalic acid product is obtained as a slurry of crystals ofterephthalic acid in a liquid phase comprising acetic acid, any catalystand impurities. The terephthalic acid may be separated from the liquidfor example by centrifugation or filtration, washed with acetic acid,dried to remove acetic acid, dissolved in water at elevated temperature,hydrogenated to reduce organic impurities and recrystallised from thewater to give a pure product.

This procedure requires drying, handling, storage and subsequentreslurrying of the crude terephthalic acid powder; all these operationsinvolve special equipment and incur both capital and variable costs ofoperation. Furthermore, such a procedure allows catalyst residues andimpurities to pass into the later stages of the process.

The present invention provides a significantly improved washing of theterephthalic acid in the reaction medium. The improved wash allowsre-optimisation of the oxidation step such that higher impurity levelsmay be tolerated in the oxidation step whilst maintaining an acceptableterephthalic acid product purity.

This invention provides a process for the production of terephthalicacid comprising oxidising paraxylene in a reaction medium containingacetic acid to produce a slurry of terephthalic acid in the reactionmedium; depositing the slurry on a moveable band of filter material;removing reaction medium from the slurry through said band in a firstzone to produce a first wet deposit; moving said deposit on said band toa second zone in which it is washed with a first aqueous medium;removing the first aqueous medium and reaction medium present in saidfirst wet deposit through said band to produce a second wet deposit;moving said second wet deposit to a third zone; removing said second wetdeposit from said band in said third zone; admixing said second deposit,either whilst so removing it or subsequently, with a second aqueousmedium, thereby producing a slurry of terephthalic acid in the secondaqueous medium, recovering terephthalic acid from the second aqueousmedium; recovering the reaction medium removed through the band in thefirst zone and recycling at least part of the reaction medium sorecovered directly or indirectly to the oxidation step.

It is desirable that the second aqueous medium is substantially purewater in order to avoid adding impurities to the terephthalic acidproduct. Further, the terephthalic acid is preferably recovered afterfurther purification desirably in a terephthalic acid purificationplant.

In order to control the temperature and/or water content of thereaction, a mixture of acetic acid and water is suitably removed fromthe reaction medium in the oxidation step by evaporation, water isremoved from at least part of the said mixture in a distillation stepleaving acetic acid of lower water content. The acetic acid of lowerwater content may then be recycled to the reaction medium in theoxidation step.

According to a further form of the invention, at least part of the firstaqueous medium and reaction medium removed through the band in thesecond zone optionally together with at least part of the reactionmedium recovered from the first zone is returned to the oxidation stepand/or distilled, eg added to said mixture distilled as mentioned above.

The so removed first aqueous medium and reaction medium and optionallythe reaction medium recovered from the first zone are recycledindirectly, for example after evaporation to separate water and aceticacid from involatile materials, or preferably directly, to the oxidationstep.

Standing concentrations of impurities in the process may be controlledby means of a purge for example by removal of at least some of theinvolatile materials recovered after evaporation.

The water and acetic acid separated in evaporation is preferablydistilled optionally together with water and acetic acid of the streamremoved from the oxidation step by evaporation, to produce acetic acidhaving a lower water content. Acetic acid having a lower water contentmay be passed to the oxidation step and the water recovered indistillation may be used as the first and/or second aqueous medium.Optionally at least part of the water recovered in distillation is usedin other steps of the process and, if present in a purification plant.

The improved washing procedure of the invention permits higher impuritylevels in the reaction medium in the oxidation step thus permitting therecycling of reaction medium from the first zone and/or the firstaqueous medium and reaction medium from the second zone andconsequently, a reduced purge is possible. A reduced purge isadvantageous as desired materials, for example terephthalic acid,terephthalic acid precursor compounds and catalyst residues my beretained in the process.

The improved wash allows a slurry of terephthalic acid in the secondaqueous medium having a lower level of impurities for example catalystresidues to be produced. Desirably this slurry is fed to a purificationplant and the efficiency of the purification plant may consequently beimproved. The lower level of impurities in the slurry allows motherliquor recovered from the purification plant, for example from theprecipitation of purified terephthalic acid product, to be recycled,hence reducing the purge amount of waste effluent to be treated whilstmaintaining an acceptable level of product purity. Furthermore, therecycled mother liquor may be used, optionally after further treatmentfor example cooling, filtering, distillation and/or evaporation, to washthe first wet deposit in the second zone. This allows a reduction in theamount of fresh water required for a given rate of wash or substantialelimination of fresh water intake.

The removal of the requirement for a separate drying stage and theexpensive equipment required for it and the associated solids handlingand storage provides a significant economic advantage in that capitalcosts may be reduced.

The band is suitably a metal gauze, or a cloth comprising a plasticsmaterial eg formed from polyester or polypropylene fibres. The band issuitably a continuous band which is moved continuously or intermittentlyto convey material comprising terephthalic acid from the first zone tothe third zone through the second zone. The second zone suitablycomprises a succession of stages in which, in each stage (other than thelast), the incoming aqueous medium passed through the solids and theband is the aqueous medium which has passed through the solids and theband in the succeeding stage. In the last stage the aqueous medium ispreferably fresh incoming water. This fresh incoming water is suitablyat least in part water separated from acetic acid in the aforesaiddistillation and/or evaporation stage or derived from other waterstreams within the process. This is advantageous as it reduces furtherthe intake of fresh water and disposal of water in the process.

In the third zone the deposit may be scraped off the band, but it ispreferably washed off with an aqueous medium, preferably substantiallypure water, which may be in the form of jets of liquid at the end of theband. In the case of a continuous band, it is desirable to providesuitable means to pass liquid for example water or alkaline solution,through the returning part of the band to wash off downwardly facingadhering deposits into a receiver.

Desirably, there is a pressure differential across the moveable band,with the side of the band on which the slurry is deposited being at ahigher pressure than the other side of the band. Suitably thedifferential pressure is at least 0.05 bar absolute and no more than thepressure at which the oxidation step is carried out, for example 30 barsabsolute. Preferably the pressure differential is 0.1 to 15 barsabsolute, more preferably, 0.2 to 7 bars absolute and especially 0.3 to3 bars absolute, for example 0.6 bar absolute. The actual pressure onthe lower pressure side of the band is maintained at such a pressurethat the reaction medium and the aqueous medium removed through the bandin the second zone remain substantially in the liquid phase.

Suitably the higher pressure side of the band is at substantially thesame pressure or a higher pressure than the preceding step in theprocess, for example a crystallisation step or the oxidation step.

The slurry of terephthalic acid in acetic acid is suitably deposited onthe moveable band at a temperature of at least 60° C. and preferably 70°to 200° C., especially 80° to 150° C. Suitably the slurry is depositedat a temperature which is sufficiently high that the pressure on thelower pressure side of the band is not less than 1 bar absolute.

Deposition of the slurry at elevated temperature is advantageous asimproved filtration is possible due to the reaction medium being lessviscous at elevated temperature. Furthermore there is lessco-crystallisation of impurities for example 4-carboxybenzaldehyde, withthe terephthalic acid product at elevated temperature. Thus a higherpurity crude terephthalic acid product is obtained and there is acorrespondingly higher level of impurities for example4-carboxybenzaldehyde in the reaction medium. This allowsre-optimisation of oxidation step reaction conditions such that variablecosts may be reduced.

The elevated temperature may also improve heat recovery and henceprovide a further reduction in variable costs. Furthermore, if apurification stage is employed, the slurry of terephthalic acid in thesecond aqueous is typically heated to dissolve the terephthalic acid;filtration at elevated temperature provides a slurry of terephthalicacid at elevated temperature therefore the amount of energy required tosubsequently heat the slurry to dissolve the terephthalic acid isreduced.

The other individual steps of the process can be carried outconventionally. The liquid reaction medium normally comprises acatalyst, for example a cobalt/manganese/bromide catalyst system whichis soluble in the reaction medium. Suitably the oxidation is carried outin the presence of an oxygen source for example air, at a pressure of 5to 30 bars absolute, and preferably an oxygen concentration of 0 to 8%by volume in the gas leaving the reactor and at a temperature of 150° to250° C. It is suitably a continuous process, and is preferably carriedout in a stirred reactor. The reaction is exothermic and the heat of thereaction may conveniently be removed by evaporation of water and aceticacid from the reaction medium.

Suitably the slurry of the terephthalic acid product in the secondaqueous medium is purified by a process which comprises dissolving theterephthalic acid in the second aqueous medium to produce a solutioncomprising terephthalic acid, contacting, under reducing conditions, thesaid solution with hydrogen and a heterogeneous catalyst for thereduction of at least some impurities, cooling the solution toprecipitate solid purified terephthalic acid product and recovering thesaid product from the solution.

Suitably the heterogeneous catalyst employed in the purification of thecrude terephthalic acid product may be a supported noble metal catalyst,for example platinum and/or preferably palladium on an inert, forexample carbon, support. The reduction is suitably carried out bypassing the aqueous solution comprising terephthalic acid andimpurities, for example 4-carboxybenzaldehyde, through a flooded bed ofcatalyst as a temperature of 250° to 350° C. in the presence ofhydrogen. The solution suitably comprises 20 to 50% by weight ofterephthalic acid.

The solution after reduction is suitably cooled to a temperature in therange 100° to 250° C. to separate pure terephthalic acid product fromthe solution. This solution is preferably subsequently cooled to atemperature in the range 15° C. to 100° C. or evaporated to produce aless pure precipitate and a mother liquor. The less pure precipitate issuitably separated from the mother liquor. The mother liquor from thisseparation my be recycled directly or indirectly to distillation and/orbe used as the second aqueous medium to reslurry the crude terephthalicacid. If desired the less pure precipitate may be recycled to theoxidation step.

Alternatively, if further purification is not employed, the terephthalicacid may be removed from the second aqueous medium, dried, for exampleby contacting it with a flow of inert gas. The dry terephthalic acidproduct may then used for further downstream processing. Optionally acrystallisation section may be employed to increase the yield of theterephthalic acid from the mixture of the second aqueous medium andterephthalic acid.

BRIEF DESCRIPTION OF THE DRAWINGS

One form of the invention will now be described with reference to theaccompanying drawing, FIG. 1, which shows a flowsheet according to theinvention and FIG. 2 which is a schematic representation of a filter andreslurry tank for use in the invention.

Reactor A is fed with paraxylene and acetic acid containing a dissolvedcatalyst comprising cobalt, manganese and bromine ions by line 1 andwith air via line 2. Product from the reactor A is passed tocrystallisation section B by line 3. The temperature within the reactorA is controlled by evaporating a mixture of acetic acid and water fromthe reactor to a condensing system C via line 4. Most of the condensateis returned to the reactor A via line 5 with noncondensibles venting vialine 6. In order to control the water content of the reactor A, part ofthe condensate is removed from the condensing system via line 7 andpassed to the distillation column D.

In the crystallisation section B the temperature is dropped toapproximately 75° C. to 120° C. and the slurry containing crystallineterephthalic acid in mother liquor thereby produced is passed to filterstage E. Acetic acid may be recovered from crystallisation section B viastreams 8 and 9 to the distillation column D and/or via streams 8 and 10and/or 11 to the reactor A.

The filter stage E and reslurry tank F are shown in FIG. 2.

FIG. 2 shows a schematic representation of a continuous band filter Eadapted for use in the invention, which comprises filter band 100 drivenby rollers within the band (not shown) and enclosed in vapour tighthousing 101 leading, by way of chute 102 to stirred reslurry tank F.Dotted lines show the locations of a first zone on the left in which aslurry of terephtalic acid and acetic acid together with any dissolvedcatalyst is introduced via line 103 onto the band and acetic acid drainsthrough the band into collector pan 104 from which it is removed vialine 105, a second (middle) zone in which water is introduced via line106 and passed through the band three times to collector pans 107, 108and 109 in turn thus washing deposits on the band with increasinglypurer water as they pass each of the three washing stages providedthereby, and a third zone in which water is sprayed on and/or throughthe band to dislodge deposits on it into chute 102, leading to tank F,which may be fed with additional water if desired from lines 14, 15, 16and/or 17. The water from line 106 may be derived from line 14, 15 orany other suitable source.

Mother liquor recovered from this stage is returned in part via line 10to the reactor A optionally by first mixing with the fresh catalyst,paraxylene and acetic acid contained in line 1. Any remaining motherliquor and the wash liquid from the second zone is suitably passed to anevaporation stage G in which water and acetic acid vapour is removed byline 11, condensed and passed to reactor A or optionally passed todistillation column D and a purge of by products and catalyst iswithdrawn via stream 13.

In reslurry vessel F the crystals are reslurried with water recoveredfrom the distillation column D via stream 14 and/or other water whichmay be recycle mother liquor via stream 15, recycle mother liquor viastream 16 and demineralised water via stream 17.

The slurry produced in this stage is heated in section H to atemperature of for example 250° C. to 350° C. to form a solution whichis passed via stream 19 to reactor J in which it is reacted withhydrogen over a fixed bed palladium catalyst thus reducing impurities inthe solution and then again crystallised in crystallisation section Kfrom which pure product is separated and dried in stage L which maycomprise centrifuges and/or filters and a drier.

The temperature to which the solution is cooled in the crystallisationsection K and the rapidity of cooling is adjusted to produce theappropriate purity of the desired terephthalic acid product. The pureterephthalic acid product is recovered from stage L and the motherliquor from the separation is passed to recovery stage M in which theliquid is evaporated or further cooled so as to permit the recovery offurther solids which my be passed back to reactor A via stream 20. Instage M the temperature of the liquor may be reduced to 100° C. byflashing steam from it at atmospheric pressure. Such steam may befurther purified for example by distillation and used if desired as washin stage L, used elsewhere in the process or purged. The remainingliquor may be cooled or evaporated further and solids separated from it.The mother liquor recovered from stage M may be in part passed back tothe distillation column D and processed as described later and may inpart be returned to the reslurring stage F via stream 16 and may in partbe purged via stream 21. Preferabling if evaporation is used theevaporated water is returned to the reslurry stage F.

The distillation column D fractionally distills a mixture of water andacetic acid evaporated from the reaction medium and is modified ifrequired for use for the treatment of mother liquor separated from stageM.

The column D comprises three zones; the upper Zone 1 comprises forexample 5 theoretical stages, the middle Zone 2 comprises for example 45theoretical stages and the lower Zone 3 comprises for example 5theoretical stages.

Part of the mixture of acetic acid and water evaporated from thereaction stage of oxidising p-xylene to terephthalic acid in reactor Ais passed via stream 7 optionally together with stream 11 via line 9 tobetween the middle and lower zones of the column D. Mother liquor fromthe precipitation of terephthalic acid may be passed into the column Dbetween the upper and middle zones via stream 22. Acetic acid and heavymaterial are passed from the base of the column D via stream 23 toreactor A. Water is condensed in the condenser and may be re-used in theprocess via stream 14.

The invention will now be illustrated by the following non-limitingexamples.

EXAMPLE 1

Samples of crude terephthalic acid in reaction medium (crude slurry)were obtained from a commercial scale oxidation plant. The samples werefiltered by a procedure which simulates a stage-wise counter currentwashing and filtration process.

A sample of crude slurry was filtered at 80° C. through a Buchner flaskthrough which a vacuum was drawn using a filter cloth which was clampedacross a supporting grid and arranged over the Buchner flask. Acylindrical vessel was clamped over the filter cloth to act as areservoir for the sample and to retain the deposit of solids material.

The sample was washed with demineralised water and the filtrate waslabelled F1(i). The deposit was discarded. A second sample was filteredand washed with F1(i) and the filtrate obtained was labelled F2(i). Thesample was then washed with demineralised water and this filtrate waslabelled F1(ii). The deposit was discarded. This procedure was repeatedwith a new sample wherein washing with F2(i) provided filtrate F3(i),washing with F1(ii) provided filtrate F2(ii) and washing withdemineralised water provided F1(iii). The deposit was discarded.

A new sample was washed successively with with F3(i) (which providedfiltrate F4(i) which was discarded), F2(ii) (which provided filtrateF3(ii)), F1(iii) (which provided filtrate F2(iii)) and demineralisedwater (which provided filtrate F1(iv)). This final washed deposit wasanalysed to determine the level of cobalt and manganese catalystresidues in it. By comparison with the known level of those residues inthe sample slurry it was possible to determine the efficiency of thewashing procedure.

Filtrate F4 represented the final wash filtrate in a 4 stagecounter-current wash. Once the filtrates representing the filtratesobtained from the first (F1), second (F2), third (F3) and fourth (F4)stages of a 4 stage counter-current wash process had been generated theywere used to wash a new sample and thereby produced filtrates F1-4 forthe next wash cycle.

The washing procedure was repeated with a fresh sample until the levelof impurities in deposits obtained from successive wash cycles remainedsubstantially constant

The results are shown in Table 1.

EXAMPLE 2

The procedure of Example 1 was repeated to simulate a 3 stagecounter-current wash procedure, that is, the filtrate labelled F3 wasdiscarded and the deposit obtained in the production of F3 was analysedto determine levels of cobalt and manganese residues.

The results are shown in Table 1.

EXAMPLE 3 (COMPARATIVE)

A conventional washing and drying procedure, not according to theinvention, was employed. Samples of crude terephthalic acid in reactionmedium (crude slurry) identical in composition to those used in Examples1 and 2 (obtained from the same source at the same time) were filteredusing a commercially available vacuum filter and washed with aceticacid. The samples were dried using a commercially available vacuum steamtube drier. The dried deposits were analysed to determine the level ofcobalt and manganese in them according to the same method as thatemployed in Examples 1 and 2. The filtration and washing was carried outat 80° C. and at the same differential pressure as the washings inExamples 1 and 2.

The results are shown in Table 1.

                  TABLE 1                                                         ______________________________________                                                               Removal  Manga-                                                                              Removal                                 Wash Rate      Cobalt  (weight  nese  (weight                                 (te/te deposit)                                                                              (ppm)   %)       (ppm) %)                                      ______________________________________                                        Example 1                                                                            0.30 (water)                                                                              0.3     99.90  2.1   99.65                                 Example 2                                                                            0.21 (water)                                                                              0.5     99.85  3.0   99.50                                 Example 3                                                                            0.21 (acetic                                                                              7.0     96.6   25.0  96.6                                  (Compar-                                                                             acid)                                                                  ative)                                                                        ______________________________________                                    

These results illustrate that a process according to the invention hasan improved wash efficiency as compared with a conventional prior artwashing process.

We claim:
 1. In a process for the preparation of terephthalic acidcomprising oxidising paraxylene in a reaction medium comprising aceticacid to produce a slurry of crude terephthalic acid in acetic acid-basedmother liquor, separating the crude terephthalic acid from the aceticacid-based mother liquor, dissolving the crude terephthalic acid inwater to produce an aqueous solution of crude terephthalic acid,purifying the terephthalic acid by contacting the aqueous solution withhydrogen and a heterogeneous catalyst, and separating the purifiedterephthalic acid from the aqueous mother liquor, the improvementwherein:separation of the crude terephthalic acid from the aceticacid-based mother liquor is effected by means of an integratedseparation and multistage countercurrent washing filter operation underelevated temperature conditions; and said integrated separation andcountercurrent washing filter serves initially to filter the aceticacid-based mother liquor from the crude terephthalic acid and thenreplace residual acetic acid-based mother liquor with water-based washliquor including aqueous mother liquor separated from the purifiedterephthalic acid.
 2. A process as claimed in claim 1 in which theaqueous mother liquor employed as wash liquor in said countercurrentwashing filter is supplied to said filter in untreated form.
 3. Aprocess as claimed in claim 1 in which the aqueous mother liquoremployed as wash liquor in said countercurrent washing filter issupplied to said filter in treated form.
 4. A process as claimed inclaim 3 in which treatment of said aqueous mother liquor comprisescooling thereof following separation from the purified terephthalicacid.
 5. A process as claimed in claim 4 in which further less pureterephthalic acid is precipitated from the aqueous mother liquor oncooling and said less pure precipitate is supplied to the oxidationreaction.
 6. A process as claimed in claim 3 in which treatment of saidaqueous mother liquor comprises distillation thereof.
 7. A process asclaimed in claim 6 further comprising controlling the temperature of theoxidation reaction by withdrawing as vapour a mixture of acetic acid andwater from the reaction, removing water from at least part of themixture in a distillation column leaving acetic acid of lower watercontent, and returning acetic acid from the distillation column to thereaction, said treatment of the aqueous mother liquor being effected insaid distillation column.
 8. A process as claimed in claim 7 in whichtreatment of said aqueous mother liquor comprises cooling thereof withaccompanying precipitation of less pure terephthalic acid, followed bydistillation of the cooled mother liquor, said less pure precipitatebeing supplied to the oxidation reaction.
 9. A process as claimed inclaim 8 in which said integrated separation and countercurrent washingfilter comprises a band of filter material.
 10. A process as claimed inclaim 8 in which said integrated separation and countercurrent washingfilter comprises a continuous band of filter material.
 11. A process forthe production of terephthalic acid comprising:supplying to a firstreaction zone paraxylene, acetic acid, a dissolved catalyst comprisingcobalt, manganese and bromine ions, and oxygen to effect oxidation ofparaxylene to terephthalic acid; controlling the temperature of thereaction by withdrawing from the reaction zone as vapour a stream ofacetic acid and water; supplying part of the acetic acid and waterwithdrawn from the reaction zone to a distillation zone and separatingwater from acetic acid in said distillation zone; returning the aceticacid so separated to the reaction zone; withdrawing from the reactionzone a crude slurry of crude terephthalic acid in acetic acid-basedmother liquor; providing an integrated separation and countercurrentwashing filter zone supplied with water-based washing liquor andfiltering the crude terephthalic acid from the acetic acid-based motherliquor in said filter zone to form a filter cake and replacing residualacetic acid-based mother liquor in the filter case with water-basedliquor; dissolving the washed crude terephthalic acid in water toproduce an aqueous solution of crude terephthalic acid; purifying thecrude terephthalic acid in a second reaction zone by contacting theaqueous solution of crude terephthalic acid with hydrogen and aheterogeneous catalyst to effect reduction of impurities associated withthe crude terephthalic acid; separating the purified terephthalic acidfrom water-based mother liquor; and feeding water-based mother liquor soseparated to said integrated separation and countercurrent washingfilter for use as said water-based liquor.
 12. Process as claimed inclaim 11 which includes the step of feeding water derived from thedistillation column to said filter zone as water-based liquor. 13.Process as claimed in claim 12 in which the integrated separation andcountercurrent washing filter zone comprises a filter band on which theseparation and washing steps are carried out.
 14. Process as claimed inclaim 13 in which the filter band is continuous.
 15. Process as claimedin claim 14 in which the filter band comprises a plastics material.